In recent years, attention has been drawn to a technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements for image display devices, in particular, is being pushed. Various metal oxides are used for a variety of applications.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics are a tungsten oxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A thin film transistor in which such a metal oxide having semiconductor characteristics is used for a channel formation region is known (Patent Documents 1 and 2).
Further, there is a description that a zinc oxide, a magnesium zinc oxide, or a cadmium zinc oxide is used as a semiconductor in Patent Document 3.